1. Field of the Invention
This invention relates to the control of power supplied to a fluorescent light fixture to achieve variable light output and, more particularly, to the control of AC power delivered to a compact fluorescent light fixture having an electronic ballast.
2. Description of the Related Art
Dimmable lights are desirable in many applications including the home, office, theater, etc. Dimmable lights permit the light intensity in a particular space to be varied to suit the immediate purpose for that room. Various methods of achieving variable light levels have evolved over the years. Many of these methods are based on the principle that the illumination intensity of a lighting load will vary proportionally to the amount of power delivered to the load.
The most common prior art systems consist of AC power regulating circuits in which AC power to a load is regulated through control of an electronic switch (e.g., a thyristor, a triac or SCR) interconnecting the source of AC power and the load. These systems typically vary the amount of power delivered to the load by advancing or retarding the time relative to a zero crossing that the electronic switch is triggered into conduction during each half cycle of the AC power cycle. Thus, the device is turned on later in the cycle to reduce power and earlier to increase power. Once activated, the electronic switch typically turns off automatically when the current through the device drops to zero, thereby blocking current to the load when the supply current reverses direction. To decrease or increase power to the load, the trigger phase angle is advanced or retarded and the portion of each half wave of AC input power that is applied to the load through the switch is thereby decreased or increased. These common prior art AC power control circuits are also referred to as phase control dimmers.
This type of power regulation circuit results in conduction occurring primarily during the later part of each half cycle of the AC power. The negative effects of these circuits include:
1. an inductive (lagging) power factor;
2. harmonic distortion;
3. noise spikes reflected into the power line; and
4. decreased power line efficiency.
Such lagging power factors frequently result in increased electric utility rates to the user. Because current does not flow from the AC source during the time the electric switch is off, substantial harmonic distortion and noise is reflected into the power line which can interfere with the operation of sensitive electronic equipment. In addition, current from the AC power line to the load is interrupted during a substantial portion of each AC half cycle, which can result in large surge currents. Large surge currents can cause ballast temperature to rise excessively, causing early failure or actual breakdown with acrid smoke generation.
The problems just described with reference to the most common prior art dimming circuits make them incompatible with many compact fluorescent light fixtures. The ballasts of compact fluorescent lamps are known as reactive loads, i.e., loads that resist rapidly changing currents. Most such fluorescent fixtures are labeled xe2x80x9cnon-dimmablexe2x80x9d.
While fluorescent light fixtures have been commonly used to light large spaces, i.e., offices and warehouses, many residential light fixtures are configured around the incandescent bulb. Homeowners enjoy the warm light, low cost, dimmability and compact size of incandescent light fixtures.
Two trends are converging to increase the use of compact fluorescent lamps configured to directly replace incandescent light bulbs. First, the drawbacks associated with fluorescent light fixtures, e.g., cold-looking light, blinking, awkward sizes and high-pitched noise, have largely disappeared. Modern compact fluorescent light bulbs are convenient, reliable and configured to fit most lamps and fixtures. Second, as electricity becomes more expensive, the efficiency inherent in a fluorescent lamp becomes more important. The most common form of compact fluorescent lamps incorporate xe2x80x9cnon-dimmablexe2x80x9d ballasts.
Many attempts have been made to produce a dimmable fluorescent light fixture. Most prior art dimmable fluorescent fixtures incorporate complex ballasts that react to a control signal to increase or reduce the power provided to the magnetic ballast. Electronic ballasts are also available which work essentially on the same principle. More expensive xe2x80x9cdimmablexe2x80x9d electronic ballasts are also available. Complex and/or electronic ballasts may be appropriate for light fixtures in which the fluorescent lamp itself is replaced independently of the ballast. However, in most compact fluorescent lamps, the ballast is manufactured as part of the lamp. Dimmable electronic ballasts increase the cost of each compact fluorescent lamp because the expensive ballast is disposed with each used compact fluorescent lamp.
There is a need in the art to provide an alternative form of AC power control that is compatible with a xe2x80x9cnon-dimmablexe2x80x9d compact fluorescent lamp. Such an AC power control should ideally be compatible with all compact fluorescent and incandescent lamps.
One embodiment of a dimmer for compact fluorescent lamps in accordance with the present invention comprises an electronic circuit including a zero-cross detector used to trigger an electronic switch to provide pulses of alternating current to the compact fluorescent lamp. The dimmer includes a current-sensing circuit to limit the amount of current applied to the compact fluorescent lamp. A low-pass filter suppresses switching noise produced by the electronic switch and smooths the flow of current to the ballast of a compact fluorescent lamp.
A dimmer in accordance with the present invention is a direct replacement for the conventional phase-control dimmer switch, requiring no additional wiring. A variable resistance (rheostat) permits the user to adjust the duration of each current pulse. Long current pulses deliver more power to the ballast of the compact fluorescent lamp and result in a brighter light, while shorter current pulses deliver less power and result in less light output.
The dimmer provides a leading power factor by initializing conduction at the zero crossing. Initializing conduction at the zero crossing of each AC half cycle allows current to build gradually in the ballast. This prevents large surge currents and damage to the ballast.
It is an object of the present invention to provide a new and improved dimmer that is compatible with xe2x80x9cnon-dimmablexe2x80x9d ballast compact fluorescent lamps.
Another object of the present invention is to provide a new and improved dimmer for compact fluorescent lamps that does not require special wiring or complicated installation.
A further object of the present invention is to provide a new and improved dimmer for compact fluorescent lamps that is of low cost and efficient construction.
These and other objects, features and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the preferred embodiments, in conjunction with the accompanying drawings.